1. Field of the Invention
The present invention generally relates to microwave amplifiers and, more particularly, to a microwave amplifier implemented by a transistor for amplifying signals having a millimeter-wave frequency or a microwave frequency.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a microwave amplifier according to the related art disclosed, for example, in Japanese Laid-Open Patent Application No. 61-285811. Referring to FIG. 4, the microwave amplifier comprises a field-effect transistor 1 having a relatively high operating frequency, a grounding terminal 2 for the field-effect transistor 1, a stabilized resistor 3 connected to the grounding terminal 2, a grounding conductor pattern 4 having one end thereof connected to the stabilized resistor 3 and the other end grounded, and an open-circuit stub 5 having a length equal to xc2xc of a wavelength at the operating frequency of the field-effect transistor 1 and connected to the grounding terminal 2 so as to be parallel with a series circuit formed of the stabilized resistor 3 and the grounding conductor pattern 4.
A description will now be given of the operation of the microwave amplifier of FIG. 4.
The microwave amplifier shown in FIG. 4 operates such that a drain current from the drain D to the source S of the field-effect transistor 1 is amplified in accordance with a gate voltage applied to the gate G.
The grounding conductor pattern 4 is a channel by which the grounding terminal 2 of the field-effect transistor 1 is grounded. In a low-frequency band, the inductance of the grounding conductor pattern 4 is negligible so that the field-effect transistor 1 is properly grounded. In a high-frequency band, the inductance of the grounding conductor pattern 4 is not negligible. The grounding conductor pattern 4 thus acts as a short-circuited stub having inductance, resulting in a loss of the gain of the field-effect transistor 1 due to the inductance of the grounding conductor pattern 4.
Accordingly, the open-circuit stub 5 having a length equal to xc2xc of the wavelength at the operating frequency of the microwave amplifier is connected to the grounding terminal 2 to provide high-frequency grounding of the grounding terminal 2 at the operating frequency.
However, at certain points in the high-frequency band including the operating frequency, the inductance of the grounding conductor pattern 4 and the capacitance of the open-circuit stub 5 produce parallel resonance, causing the reactance of the grounding terminal 2 to become infinite at the parallel resonance frequency.
The stabilized resistor 3 is connected between the grounding terminal 2 and the grounding conductor pattern 4 in order to suppress parallel resonance at the parallel resonance frequency.
FIG. 5 is a circuit diagram of a microwave amplifier according to the related art shown in TECHNICAL REPORT OF IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) MW 92-149 LOW-NOISE AMPLIFIER USING DIRECTLY COOLED HEMTs, FEBRUARY, 1993. Referring to FIG. 5, the microwave amplifier includes the field-effect transistor 1, the grounding terminal 2 of the field-effect transistor 1, and the inductor 7 having one end thereof connected to the grounding terminal 2 and the other end grounded.
A description will now be given of the operation of the microwave amplifier of FIG. 5.
The microwave amplifier shown in FIG. 5 operates such that a drain current from the drain D to the source S of the field-effect transistor 1 is amplified in accordance with a gate voltage applied to the gate G.
The inductor 7 can be configured such that the impedance that minimizes the noise for the field-effect transistor 1 substantially matches the impedance that minimizes reflection. Thus, the noise characteristic and the reflection characteristic can be simultaneously improved.
The microwave amplifier of FIG. 4 has a drawback in that the stabilized resistor 3 produces a voltage drop when a bias is applied to the field-effect transistor 1. This makes it difficult to use the construction as shown in FIG. 4 in a high-power amplifier having a large current consumption.
The microwave amplifier of FIG. 5 has a drawback in that when a member having inductance and capacitance is used to implement the inductor 7, parallel resonance results at a certain frequency (parallel resonance frequency) so that the reactance of the grounding terminal 2 becomes infinity at the parallel resonance frequency. If this occurs, the operation of the microwave amplifier becomes unstable.
Accordingly, an object of the present invention is to provide microwave amplifiers in which the aforementioned drawbacks are eliminated.
Another and more specific object of the present invention is to provide a microwave amplifier optimized for stable operation.
The aforementioned objects can be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; a grounding conductor pattern having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a first open-circuit stub having a length equal to one quarter of a wavelength at an operating frequency of said transistor and connected to the grounding terminal of said transistor so as to be placed in parallel with said grounding conductor pattern; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said grounding conductor pattern; and a second open-circuit stub having a length equal to one quarter of a parallel resonance frequency of said grounding conductor pattern and said first open-circuit stub, and connected to the other end of said resistor.
Accordingly, it is not only possible to suppress undesirable parallel resonance caused by the inductance of the grounding conductor pattern and the capacitance of the first open-circuit stub, but also to prevent a voltage drop in the stabilized resistor when a bias is applied to a transistor. Thus, a microwave amplifier optimized for stable operation and applicable to a high-power amplifier having a large current consumption is obtained.
The aforementioned objects can also be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; an inductor having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said inductor; and an open-circuit stub having a length equal to a quarter of a parallel resonance frequency of an inductance of said inductor and a capatitance inherent in a member constituting an inductor, and connected to the other end of said resistor.
Accordingly, it is not only possible to suppress undesirable parallel resonance caused by the inductance of the grounding conductor pattern and the capacitance of the first open-circuit stub, but also to prevent a voltage drop in the stabilized resistor when a bias is applied to a transistor. Thus, a microwave amplifier optimized for stable operation and applicable to a high-power amplifier having a large current consumption is obtained.
The aforementioned objects can also be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; an inductor having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said inductor; and an open-circuit stub having a length equal to a quarter of a parallel resonance frequency of an inductance of said inductor and a capatitance inherent in a member constituting an inductor, and connected to the other end of said resistor.
Accordingly, undesirable parallel resonance caused by inductance and capacitance of a member constituting an inductor is suppressed so that the stable operation of a microwave amplifier is achieved.